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A Year At RHIC First Data, New Phenomena

RHIC is an unprecedented hadron collider providing a superb environment for studying QCD as a fundamental theory. The experiments at RHIC cover a broad range of phenomena such as phase transitions, confinement, chiral symmetry breaking, and the origin of proton spin. Initial results show indications of new phenomena and excellent agreement between the experiments. Join us on this exciting journey of discovery!

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A Year At RHIC First Data, New Phenomena

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  1. A Year At RHICFirst Data, New Phenomena W.A. ZajcColumbia University W.A. Zajc

  2. Conclusions • RHIC is a hadron collider of unprecedented versatility. • The four RHIC experiments have a broad coverage ideally suited to exploit RHIC’s potential. • RHIC and its experiments provide a superb environment for the study of QCD as a fundamental theory: • Phase transition(s) associated with • Confinement • Chiral symmetry breaking • Origin of proton spin • Initial results from RHIC show superb agreement between the experiments and indications of new phenomena W.A. Zajc

  3. Standard Model

  4. Probes: • ~all of the allowed hadrons that can be formed from those quarksp, K, h, r, w, p, n, K*, f, L, X, W, D, d, J/Y,… • (eventually)“non-interacting” produced photons, both real and virtualg, e+e-, m+m- Relevant Parts of SM • Players: • Quarks (u, d, s, eventually c, b) with any of three color charges (r,g,b) with gauge couplings that generate interactions via exchange of • Gluons with 8 varieties of color charge W.A. Zajc

  5. QCD is not QED • QED (Abelian): • Photons have do not carry charge • Flux is not confined  1/r potential  1/r2 force • QCD (Non-Abelian): • Gluons carry charge (red, green, blue)(anti-red, anti-green, anti-blue) • Flux tubes form  potential ~ r  constant force + +… W.A. Zajc

  6. Making Something from Nothing • Explore non-perturbative “vacuum” that confines color flux by melting it • Experimental method: Energetic collisions of heavy nuclei • Experimental measurements:Use probes that are • Auto-generated • Sensitive to all time/length scales • Particle production • Our ‘perturbative’ region is filled with • gluons • quark-antiquark pairs • A Quark-Gluon Plasma (QGP) W.A. Zajc

  7. Why is RHIC? • To understand fundamental features of the strong interaction: • We have a theory of the strong interaction: • Where does the proton get its spin? • How does nuclear matter “melt”? (This works well except when the interaction is strong…) W.A. Zajc

  8. Experimental Gauge Theory • QCD is the only fundamental gauge theory amenable to experimental study in both • Weak and strong coupling limits • Particle and bulk limits • RHIC • (Large g, bulk ) limit  heavy ion collisions • (Large g, particle) limit  spin physics • (Small g, particle) limit  W’s as helicity probes • (Small g, bulk ) limit  high pT probes of plasma state

  9. NOT Here’s what RHIC (heavy ion) experiments are not : • Scattering • Rutherford, Hofstader, Kendall/Friedman/Taylor • (New) Particle Production • Positron, anti-proton, … Omega … Top … Higgs W.A. Zajc

  10. Phase Diagrams Water Nuclear Matter W.A. Zajc

  11. Relevant Nuclear Physics • Normal nuclei have ~ constant density: • r0 ~ 0.16 GeV / fm3 • MPROTON ~ MNEUTRON ~ 1 GeV • R(A) = 0.92 A1/3 (rms) , where A = AtomicNumber W.A. Zajc

  12. Energy density for “g” massless d.o.f 8 gluons, 2 spins;  2 quark flavors, anti-quarks, 2 spins, 3 colors 37 (!) “Reasonable” estimate Relevant Thermal Physics Q. How to liberate quarks and gluons from ~1 fm confinement scale? A. Create an energy density • Need better control of dimensional analysis: W.A. Zajc

  13. Pressure in plasma phase with “Bag constant” B ~ 0.2 GeV / fm3 Pressure of “pure” pion gas at temperature T Slightly More Refined Estimate • Compare • Select system with higher pressure: Phase transition at T ~ 140 MeV with latent heat ~0.8 GeV / fm3 Compare to best estimates (Karsch, QM01)from lattice calculations:T ~ 150-170 MeV latent heat ~ 0.70.3 GeV / fm3 W.A. Zajc

  14. Aside 1 • Previous approach (using a “pure” pion gas) works because QCD is a theory with a mass gap { • This gap is a manifestation of the approximate SU(2)R x SU(2)Lchiral symmetry of QCD with pions as the Nambu-Goldstone bosons W.A. Zajc

  15. requires T < TH Fit to this form with TH = 163 MeV Aside 2 The frightening density of hadronic levels led to concepts of • A “limiting temperature” TH (Hagedorn, 1965) • A phase transition(?) in hadronic matter before quarks were understood as underlying constituents W.A. Zajc

  16. 0.66 TC T =0 0.90 TC 1.06 TC Aside 3 What lattice QCD does superbly (uniquely?): Study potential V(r) = -a/r + s ras a function of temperature T F. Karsch, hep-ph/0103314

  17. g The Early Universe, Kolb and Turner Previous Attempts First attempt at QGP formation was successful (~1010 years ago) ( Effective number of degrees-of-freedom per relativistic particle ) W.A. Zajc

  18. RHIC Specifications • 3.83 km circumference • Two independent rings • 120 bunches/ring • 106 ns crossing time • Capable of colliding ~any nuclear species on ~any other species • Energy: • 500 GeV for p-p • 200 GeV for Au-Au(per N-N collision) • Luminosity • Au-Au: 2 x 1026 cm-2 s-1 • p-p : 2 x 1032 cm-2 s-1(polarized) 6 3 5 1’ 4 1 2 W.A. Zajc

  19. How is RHIC Different? • It’s a collider • Detector systematics independent of ECM • (No thick targets!) • It’s dedicated • Heavy ions will run 20-30 weeks/year • It’s high energy • Access to perturbative phenomena • Jets • Non-linear dE/dx • Its detectors are comprehensive • ~All final state species measured with a suite of detectors that nonetheless have significant overlap for comparisons W.A. Zajc

  20. 15-20% b 10-15% 5-10% 0-5% Fixing Initial Conditions • Event characterization • Impact parameter b is well-defined in heavy ion collisions • Event multiplicity predominantly determined by collision geometry • Characterize this by global measures of multiplicity and/or transverse energycorrelated with ZDC  Zero Degree Calorimeter • Goals: • Uniform luminosity monitoring at all 4 intersections • Uniform event characterization by all 4 experiments • Process: • Correlated Forward-Backward Dissociation • stot = 11.0 Barns (+/- few %) W.A. Zajc

  21. Run-1 Results • RHIC worked (i.e, achieved its Year-1 goals): • Stable operation at 130 GeV • Delivery of 10% of design luminosity • All four experiments worked • All four experiments produced quality data within a few months of initial RHIC operation • Particle yields • Rapidity and pT spectra • Flow • Source sizes (Etc.) • This from a data set equivalent to 1-3 days running of RHIC at design luminosity W.A. Zajc

  22. STAR RHIC’s Experiments W.A. Zajc

  23. 2. Initial State Role of event geometry and gluon distributions • Final State • Yields of produced particles • Thermalization, Hadrochemistry 3. Plasma(?) Probes of dense matter Outline Will present sample of results from various points of the collision process: W.A. Zajc

  24. Kinematics Dynamics Kinematics 101 Fundamental single-particle observable: Momentum Spectrum W.A. Zajc

  25. BRAHMS Acceptance (PID) Acceptances PHOBOS Acceptance STAR Acceptance W.A. Zajc

  26. STAR Event Data Taken June 25, 2000. Pictures from Level 3 online display. W.A. Zajc

  27. STAR W.A. Zajc

  28. Results on Particle Composition Anti-particle/particle ratios from PHOBOS and BRAHMS mid-rapidity Anti-particle, particle spectra from PHENIX and STAR W.A. Zajc

  29. Agreement between Experiments • Excellent agreement in common observables provides confidence • Overlaps in detector capabilities a feature of RHIC program W.A. Zajc

  30. BRAHMS An experiment with an emphasis: • Quality PID spectra over a broad range of rapidity and pT • Special emphasis: • Where do the baryons go? • How is directed energy transferred to the reaction products? • Two magnetic dipole spectrometers in “classic” fixed-target configuration W.A. Zajc

  31. Baryon number, found(?) at non-zero rapidities, must be measured away from 90 degrees • Enter BRAHMS Where do the Baryons Go? • There is a (not-quite-perfect) correspondence between longitudinal momentum (rapidity) and the angle of emission in the center-of-mass W.A. Zajc

  32. BRAHMS Results • Anti-proton to proton yields as a function of rapidity: (Two points are reflected about y=0) • Clear evidence for development of (nearly) baryon-free central region W.A. Zajc

  33. √s [GeV] Approaching the Early Universe • Early Universe: • Anti-proton/proton = 0.999999999 • We’ve created “pure” matterapproaching this value pbar/p • For the first timein heavy ion collisions, more baryons are pair-produced than brought in from initial state NA44 Pb+Pb E866 Au+Au W.A. Zajc

  34. Compilation and Figure from M. Kaneta 130 GeV RHIC : STAR / PHENIX / PHOBOS /BRAHMS 17.4 GeV SPS : NA44, WA97 Simple Chemistry • Assume • chemical description appropriate  (m,T) • Boltzmann approximation valid  n(m,T) ~ e m / T • Feed down negligible  use “raw” ratios • Then W.A. Zajc

  35. Final-state analysis suggests RHIC reaches the phase boundary • Difficult for hadrons to probe earlier than this “freeze-out” • <E>/N ~ 1 GeV(J. Cleymans and K. Redlich,Phys.Rev.C60:054908,1999 ) Locating RHIC on Phase Diagram • Baryon ratios determine m / T • K/p ratio, momentum spectra determine T • TCH = 190 ± 20 MeV, B = 45 ± 15 MeV (M. Kaneta and N. Xu, nucl-ex/0104021) W.A. Zajc

  36. t Dynamics 101 Q. How to estimate initial energy density? A. From rapidity density • “Highly relativistic nucleus-nucleus collisions: The central rapidity region”, J.D. Bjorken, Phys. Rev. D27, 140 (1983). • Assumes • ~ 1-d hydrodynamic expansion • Invariance in y along “central rapidity plateau”(I.e., flat rapidity distribution) • Then since boost-invariance of matter  where t ~ 1 fm/c W.A. Zajc

  37. Determining Energy Density EMCAL • What is the energy density achieved? • How does it compare to the expected phase transition value ? PHENIX For the most central events: Bjorken formula for thermalized energy density time to thermalize the system (t0 ~ 1 fm/c) ~6.5 fm eBjorken~ 4.6 GeV/fm3 pR2 ~30 times normal nuclear density ~1.5 to 2 times higher than any previous experiments W.A. Zajc

  38. Generally: requires number density at temperature T to exceed ~1 per “de Broglie volume” l3 • Non-relativistic: • Relativistic:(i.e., number and energy densities approach blackbody values) (Bose-Einstein) Condensates(?!?) • Note that Bose condensation • Does not require low temperature • Does require high phase space densities • Any realistic estimate of RHIC densities • Significantly (x10-100) in excess of these values • Quantum, condensate effects may be important • But -- effects in highly dynamic, interacting system difficult to quantify W.A. Zajc

  39. z Hydrodynamic limit STAR: PRL86 (2001) 402 PHOBOS preliminary (scaled) spatial asymmetry y x (PHOBOS : Normalized Paddle Signal) Compilation and Figure from M. Kaneta Centrality Dependence of Elliptic Flow Parameterize azimuthal asymmetry of charged particlesas 1 + 2 v2cos (2 f) Evidence that initial spatial asymmetry is efficiently translated to momentum space ( as per a hydrodynamic description) W.A. Zajc

  40. PHOBOS An experiment with a philosophy: • Global phenomena • large spatial sizes • small momenta • Minimize the number of technologies: • All Si-strip tracking • Si multiplicity detection • PMT-based TOF • Unbiased global look at very large number of collisions (~109) W.A. Zajc

  41. ~2A PHOBOS Result on dNch/dh Q. How many charged particles per nucleon-nucleon collision? A: Express using • dNch/dh ~ “generic” dNch/dy • Npart  Number of participating nucleons = 2 for p-p collision ~2A for central A-A collision • Plot multiplicity per N-N collision ( dNch/dh ) / ( Npart/2 ) • Does not (quite) distinguish between • “Saturation” models, dominated by gg g • “Cascade” models, dominated by gg gg, gg ggq ( X.N. Wang and M. Gyulassy, nucl-th/0008014 ) W.A. Zajc

  42. PHENIX W.A. Zajc

  43. PHENIX Results • Excellent consistency between two analyses • Yields grow significantly faster than Nparticipants • Evidence for term ~ Ncollisions • Qualitatively consistent with HIJING • Inconsistent with some saturation models Evidence for highest energy densities yet achieved (~ 5 GeV/fm3) W.A. Zajc

  44. dN/dh / .5Npart Npart Physics, Consistency between Experiments • Trend • incompatible with final-state gluon saturation model • Good agreement with model based on initial-state saturation (Kharzeev and Nardi, nucl-th/0012025) • Excellent agreement between (non-trivial) PHENIX and PHOBOS analyses of this systematic variation with nuclear overlap. W.A. Zajc

  45. Gluon saturation at RHIC / xpz 2R dT =  /Q 2R m/pz Longitudinal Transverse When do the gluons overlap significantly? 1 J.P Blaizot, A.H. Mueller, Nucl. Phys. B289, 847 (1987) So for  /mx ~ 2R , ~ all constituents contributeParton density r ~ A xG(x,Q2) /R2 Parton cross section s ~ aS2/ Q2 Saturation condition rs ~ 1 QS2 ~ aS A xG(x,Q2) /R2 ~ A1/3 D. Kharzeev, nucl-th/0107033 W.A. Zajc

  46. Initial State Partons? • Procedure: • Determine scale QS2 as function of nuclear overlap • QS2 ~ A1/3 ~ Npart1/3 • Assume final-state multiplicities proportional to number of initial-state gluons in this saturated regime: • dN/dh = c Npart xG(x, QS2) • Note that DGLAP requires evolution of xG(x, QS2) : • xG(x, QS2) ~ ln (QS2 / LQCD2) • Results: • “Running” of the multiplicity yielddirectly from from xG(x, Q2) +DGLAP W.A. Zajc

  47. pp Extensions of Saturation Approach* • Use HERA data, counting rules • x G(x,Q2) ~ x-l (1-x)4 • Describe rapidity dependence: • y ~ ln(1/x)  QS2(s,y) = QS2(s,y=0)ely • Predict energy dependence: • x = QS / s QS2(s,y) = QS2(s0,y) (s/s0) l/2 • Predict10-14% increase betweens = 130 and 200 GeV • Versus 146% reported by PHOBOS * D. Kharzeev and E. Levin, nucl-th/0108006 W.A. Zajc

  48. Energy Loss of Fast Partons • Many approaches • 1983: Bjorken • 1991: Thoma and Gyulassy (1991) • 1993: Brodsky and Hoyer (1993) • 1997: BDMPS- depends on path length(!) • 1998: BDMS • Numerical values range from • ~ 0.1 GeV / fm (Bjorken, elastic scattering of partons) • ~several GeV / fm (BDMPS, non-linear interactions of gluons) W.A. Zajc

  49. Rare processes at RHIC Both PHENIX and STAR have measured charged particle spectra out to “small” cross sections W.A. Zajc

  50. Expectations • Particle production via rare processes should scale with Ncoll, the number of underlying binary nucleon-nucleon collisions • Assuming no “collective” effects • Test this on production at high transverse momentum W.A. Zajc

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